Production of piperidine



Patented Feb. 20, 1934 UNITED STATES PATENT orslcs No Drawing.

Application February 17, 1933,

Serial No. 657,257, and in Great Britain December 24,v 1931 4 Claims.

This invention relates to the manufacture of piperidine by the electrolysis of pyridine and sulphuric acid. Such a process was first successfully employed by W. Pip and is described by him in his U. S. Patent No. 680,543 of August 13th, 19(11. The invention is based upon the discovery that the yield of piperidine can be substantially increased if certain conditions are complied with.

In the first place it is found that it is essential to use a diaphragm cell if good yields are to be obtained, and that without a diaphragm the total yield of bases is only about half of that if a diaphragm is used. The pyridine is placed in the catholyte chamber of the cell together with acid which bears a ratio to the pyridine lying within certain limits. The cell is then worked at a temperature which depends upon this acid ratio as described below. In addition it is found that the anolyte must contain acid of a certain minimum strength according to the strength of the catholyte.

The minimum ratio of acid to pyridine in the catholyte is about 2.0 equivalents of the pyridine present, because, if the acid is weaker than this, at low temperatures so large a proportion of dipiperidyls is formed as to render the process uneconomical while at temperatures sufficiently high to give a reasonable yield of liquid bases, only partial reduction takes place giving a product of low purity.

At any given temperature, increase of acid strength suppresses the formation of dipiperidyls and improves the yield, but, like increase of temperature, it also tends to prevent reduction, and the two factors must therefore be varied inversely. Thus with 2 equivalents at 80 C. one can obtain almost the same results as with 6 equivalents at C.

The temperatures to be employed are set out in the table below in which the useful temperature range is that over which the yield of piperidine is more than 75% by volume of the pyridine taken and the purity of the piperidine formed is more than 85%.

Ratiori acid to pyn me in catholyte measigg gggg ured in equivalents 2. 0 70-85 C. 3.0 80 O. 4. 0 Bil--75 C. 5. 0 2565 C. 6.0 20-50 o.

The strength of the anolyte or pot acid mainly affects the results in so far as it affects the strength of the catholyte by diffusion; passage of the current will cause an increase of the anolyte strength, while simple diffusion will tend to equalize the strength of the two acids. The preferred strength of the anolyte is such that as little change as possible takes place in the strength of the catholyte. Thus, if the anolyte is too weak, so much acid may pass from the catholyte compartment that conditionsfor the formation of excessive dipiperidyls or even gelatinous matter may be reached during electrolysis. On the other hand, if the anolyte is too strong initially the strength may increase during electrolysis to such a point that the anodes rapidly dissolve. It is found that the pot acid strength should be between about 60 and 85 Twaddell, and 70 Twaddell is preferred.

The current density can be varied over fairly wide limits, but it is preferred to employ a current density of about 13 to 15 amperes per square decimeter. It is found that the total current passed may be about equal to that required theoretically. No further reduction takes place if more current is passed, while the use of less current leaves some of the pyridine unreduced.

Pure sulphuric acid should be used in the cell to convert the pyridine into pyridine sulphate. Preferably the latter contains about 10% by volume of pyridine.

In the preferred process the cells are lined with lead, and lead cathodes in electrical contact with the cell are used, while the anodes are also of lead. Some or all of the cathodes may be of such a construction that they may be either cooled or heated in order to lead to as much uniformity of temperature throughout the whole bath as possible.

During the electrolysis the level of the acid in 5 the pots drops and should be brought up to the normal level from time to time by the addition of water, but even so it is found that the strength of the pot acid increases. After four or five runs, therefore, it is necessary to adjust the strength 9 of acid in the pots. If the process is being car ried on for only a part of the day, this may conveniently be done by allowing the pots to stand in water for about half a day, whereupon sufficient acid diffuses out into the water to bring the strength of the pot acid back to about 70 Twaddell. The dilute acid thus produced may be used for the same purpose several times until it is sufficiently strong for use in making up pyturned to normal.

ridine sulphate. This ensures that no acid is wasted.

If impurities gain access to the cell, the latter may be poisoned and the purity of the product may deteriorate and the yield drop. The cells may be cleaned in such a case by reversing the direction of flow of the current after filling the cells with fairly dilute acid. Impurities deposited on the cathodes are then dissolved and the cathodes are coated with a film of oxide. The acid used in this cleaning process may then be replaced by clean acid and the direction of the current re- When the film of oxide on the cathodes has been reduced to pure lead, the cells are ready for use. The principal commercial impurities to be avoided are iron and copper. If the cell becomes badly poisoned, it may best be cleaned with 20% nitric acid until the cathodes have a bright metallic appearance, the acid being then washed out with water. Before the cathodes can reach their normal efficiency again, however, they must be formed by alternate ox idation and reduction.

The product of the electrolysis is piperidine sulphate. This is preferably treated with liquid caustic soda in suificient quantity to react with the total acid present and is then distilled when water and piperidine pass over and are condensed. The crude piperidine thus produced is dried and preferably allowed to stand over caustic soda for a few weeks before final distillation.

I claim:

1. A process for the manufacture of piperidine by the electrolysis of pyridine and sulphuric acid in a diaphragm cell having a cathode chaniher and an anode chamber which comprises placing pyridine in the cathode chamber in such amount that 10% by volume of the catholyte is pyridine, adjusting the content of sulphuric acid in the catholyte to such an amount that it is present in the ratio of from 2.0 to 6.0 equivalents of the pyridine, regulating the sulphuric acid in the anode chamber in such an amount that the anolyte is of a density not less than Twaddell, and conducting the electrolysis at,

a temperature determined in accordance with the ratio between acid and pyridine in the catholyte as follows:

Ratio of acid to pyridine in the x catholyte meas- Tmggmtule ured in equivg8 alents 2. 0 -85 C 3. 0 4580 C 4. 0 30-75 C 5. 0 25-65 C 6. 0 2050 C 

